1. Field of the Invention
The present invention relates to an integrated optical semiconductor device and a manufacturing method thereof, and particularly to an integrated optical semiconductor device monolithically formed by opposing and joining three or more kinds of optical elements in an optical axis direction and a manufacturing method thereof.
2. Description of the Related Art
In recent years, there is increasing demand for transmitting vast amounts of information at low cost as broadband communications makes progress and a public telecommunication network using an optical fiber prevails. To increase the amounts of information transmitted according to such demand, it is necessary to improve a transmission rate. The transmission rate is gradually sped up from 600 Mbps to 2.5 Gbps and further to 10 Gbps.
Due to such improvement in the transmission rate of optical communication devices, optical communication networks expanded their markets to access systems such as offices and households in addition to trunk-line systems, and light emitting and light receiving devices used for optical transmitters and receivers are required to be high-speed, low-cost and efficient.
To configure an optical semiconductor device for optical communications at low cost, there are an optical modulator integrated semiconductor laser device, an optical amplifier integrated semiconductor laser device and the like which are monolithically integrated for instance.
These integrated optical semiconductor devices comprise a semiconductor laser, an optical semiconductor device such as an optical modulator or an optical amplifier pre-placed on the semiconductor laser by sharing an optical axis with the semiconductor laser, and a window layer structure portion pre-placed by sharing the optical axis with these optical devices.
The semiconductor laser, various optical devices and window layer structure portion configuring these integrated optical semiconductor devices are configured by opposing and joining (called butt-joint hereafter) the three different kinds of laminated structure as basic configurations of the optical devices having their respective functions to share one optical axis.
To describe the optical modulator integrated semiconductor laser device for instance, each of the semiconductor laser, the optical modulator pre-placed on (or located in front of) the semiconductor laser and the window layer structure portion pre-placed on the optical modulator has a double hetero junction structure consisting of InP/InGaAsP/InP for instance in terms of the basic configuration.
The semiconductor laser has the double hetero junction structure including an InGaAsP layer epitaxially grown on an InP substrate and an InP layer epitaxially grown on the InGaAsP layer as a basic laminated structure for instance.
The optical modulator has the double hetero junction structure including an InGaAsP layer epitaxially grown on an InP substrate and an InP layer epitaxially grown on the InGaAsP layer as the basic laminated structure for instance.
The window layer structure portion has the double hetero junction structure including an InP layer epitaxially grown on an InP substrate, an InGaAsP layer epitaxially grown on the InP layer and an InP layer epitaxially grown on the InGaAsP layer as the basic laminated structure for instance. As for the basic laminated structures of the three optical elements, each of the laminated structures is joined by butt-joint on the InP substrate which is common.
As for a heretofore known example of such a semiconductor integrated optical circuit which is monolithically integrated, there is a disclosed example which is configured by the semiconductor laser, a dilute magnetic semiconductor (DMS) layer and liquid crystal polarizers provided at both ends thereof and monolithically configures an optical isolator and an optical waveguide.
To describe the method of forming the semiconductor integrated optical circuit, it sequentially forms a DFB laser including a cladding layer consisting of a diffraction grating formed on a substrate, an InGaAsP active layer formed thereon and InP formed thereon, an optical isolator region including a DMS layer consisting of a dilute magnetic semiconductor layer of an (InGaMn) As system having the same optical axis as the DFB laser and a cladding layer consisting of InP, and an optical waveguide including an optical waveguide layer consisting of InGaAsP also having the same optical axis and a cladding layer consisting of InP, and forms concave places by etching on a boundary between a DFB laser region and the optical isolator region and a boundary between the optical isolator region and the optical waveguide so as to form the liquid crystal polarizers there (refer to paragraph numbers [0019] to [0035] and FIGS. 1 to 4 of Japanese Patent Laid-Open No. 2002-277826).
For instance, the following manufacturing method was used to form the aforementioned basic laminated structures of the three optical elements such as the optical modulator integrated semiconductor laser device having the double hetero junction structure consisting of InP/InGaAsP/InP for instance respectively.
First, a first InGaAsP layer and a first InP layer as the basic laminated structure of the semiconductor laser sequentially are formed on the InP substrate by epitaxial growth.
Next, an silicon oxide film is formed on the surface of the first InP layer, and also a first silicon oxide film pattern is formed, the first silicon oxide film pattern including an opening in a portion corresponding to a region including the window layer structure portion located at the most front end on the optical axis and of a length allowing a margin a little further in the optical axis direction. With the first silicon oxide film pattern as a mask, the first InP layer is completely eliminated by nonselective etching such as dry etching using a reactive ion for instance in the depth control based on etching time. Subsequently, selective etching is carried out by using an etchant such as tartaric acid rapidly reactive to InGaAsP and slowly reactive to InP so as to eliminate the first InP layer and the first InGaAsP layer with good etching depth control.
Next, with the first silicon oxide film pattern as the mask, a second InP layer, a second InGaAsP layer and a third InP layer as the basic laminated structure of the window layer structure portion sequentially are formed by the epitaxial growth to perform embedding growth. In this case, a first butt-joint is formed so that the basic laminated structure including the window layer structure portion and the basic laminated structure of the semiconductor laser share the optical axis.
Next, the basic laminated structure of the optical modulator portion is formed. First, the first silicon oxide film pattern is eliminated, a silicon oxide film is formed on the surfaces of the basic laminated structure including the window layer structure portion and the basic laminated structure of the semiconductor laser, and a second silicon oxide film pattern is formed, the second silicon oxide film pattern including an opening in the portion corresponding to the optical modulator located between the window layer structure portion and the semiconductor laser and sharing the optical axis with them. A part of the basic laminated structure of the window layer structure portion and a part of the basic laminated structure of the semiconductor laser formed earlier are included in the opening of the second silicon oxide film pattern. Therefore, the first butt-joint exists in the opening.
Next, the etching is performed with the second silicon oxide film pattern as the mask. As described above, the opening of the second silicon oxide film pattern includes a part of the basic laminated structure of the window layer structure portion and a part of the basic laminated structure of the semiconductor laser. Therefore, it is not possible to carry out the nonselective etching as performed on forming the window layer structure portion, which is performed by the selective etching, thereby to stop the etching with high-precision etching depth control.
To be more specific, it is possible to use the selective etching using the tartaric acid to a part of the basic laminated structure of the semiconductor laser because it has the InGaAsP layer on the InP substrate. However, it is not possible to use the selective etching using the tartaric acid to a part of the basic laminated structure of the window layer structure portion because it has the InP layer formed by the epitaxial growth existing on the InP substrate. For this reason, the nonselective etching has to be performed and etching depth has to be controlled by etching time when performing etching with the second silicon oxide film pattern as the mask.
Consequently, the etching depth is influenced by variations in reaction rate, such as variations of ±20% or so.
Therefore, in the case of forming the InGaAsP layer and InP layer as the basic laminated structure of the optical modulator on the InP substrate by the embedding growth with the second silicon oxide film pattern as the mask after the etching, a relative position of the basic laminated structure of the optical modulator on the InP substrate in a lamination direction deviates from the relative positions of the basic laminated structures of the semiconductor laser and window layer structure portion on the InP substrate.
As regards the optical modulator integrated semiconductor laser device for instance, there arises a situation where a deviation occurs between an active layer of the semiconductor laser and an absorbing layer of the optical modulator so that optical modulation is not efficiently performed. This also applies to the optical amplifier integrated semiconductor laser device, and a high-efficiency optical amplifier integrated semiconductor laser device is not formed.
Thus, the conventional manufacturing method had a problem that, in the case of monolithically forming three or more optical elements having different double hetero basic laminated structures by the butt-joint, it was difficult to integrate optical semiconductor elements without the deviations of the relative positions in a layer thickness direction from the substrate with good controllability.